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MT Transmission Process Based on the Share-MDT

MT Transmission Process Based on the Share-MDT

After the Share-MDT is established, MT transmission can be performed.

MT Transmission Process
Based on the Share-MDT

The VPN instance on a PE sends a VPN multicast packet to an
Multicast Tunnel Interface (MTI).

Regardless of whether the packet is a protocol packet or a
data packet, the PE encapsulates the packet with the MTI address being
the source address and the share-group address being the group address
and converts the packet into a public network multicast data packet.Figure 7-9 shows the encapsulation
format of a public network multicast data packet.

The PE then forwards the multicast data packet of the public
network to the public network instance. Then the public network instance
sends out the packet.

The packet is forwarded to the public network instance on the
remote PE along the Share-multicast distribution tree (Share-MDT).

The remote PE decapsulates the packet, reverts it to a VPN
multicast packet, and forwards it to the VPN instance.

Figure 7-9 shows
the process of converting a VPN multicast packet into a public network
multicast data packet and then into a VPN multicast packet. Table 7-1 describes the meaning
of each field in a VPN or public network multicast packet.

Figure 7-9 Process of converting a VPN multicast packet

Table 7-1 Fields in a VPN and public network multicast packet

Field

Description

C-IP Header

IP header of a VPN multicast packet.

C-Payload

Type of a VPN multicast packet, which can be a protocol
or data packet.

GRE

Generic Routing Encapsulation (GRE) encapsulation.

P-IP Header

IP header of a public network multicast data packet. In
this header, the source address is the MTI interface's address,
and the destination address is the share-group's address.

Major Tasks in the MT
Transmission Process

MTIs exchange Hello packets to set up PIM neighbor relationship
between VPN instances on each PE.

MTIs exchange other protocol packets to set up a VPN MDT.

The MT transmits VPN multicast data.

NOTE:

All interfaces that belong to the same VPN, including the PE
interfaces bound to the VPN instance and MTI, must be in the same
PIM mode.

The VPN instance and the public network instance are independent
of each other. They can be in different PIM modes.

Process of Transmitting
Multicast Protocol Packets

When a VPN runs PIM-DM,

MTIs exchange Hello packets to set up PIM neighbors.

Flooding-pruning is initiated across the public network to
create a shortest path tree (SPT).

When a VPN runs PIM-SM,

MTIs exchange Hello packets to set up PIM neighbors between
VPN instances.

If receivers and the VPN Rendezvous Point (RP) belong to different
sites, receivers need to send Join messages across the public network
to set up a shared tree.

If the multicast source and the VPN RP belong to different
sites, registration must be initiated across the public network to
set up a source tree.

In the following example, the public network and VPNs run
PIM-SM. VPN receivers send Join messages across the public network.
An example is given to show the process of transmitting multicast
protocol packets.

As shown in Figure 7-10, the receiver in VPN A belongs
to Site2 and is connected to CE2. CE1 is the RP of the VPN group G
(225.1.1.1) and belongs to Site1.

Figure 7-10 Process of transmitting multicast protocol packets

The process of exchanging multicast protocol packets is as
follows:

Through IGMP, the receiver informs CE2 to receive and forward
data of the multicast group G. CE2 creates the (*, 225.1.1.1) entry
locally, and then sends a Join messages to the VPN RP (CE1).

The VPN instance on PE2 receives the Join message sent by CE2,
creates the (*, 255.1.1.1) entry, and specifies an MTI as the upstream
interface. The instance then forwards the Join message to the P for
further processing. The VPN instance on PE2 then considers that the
Join message is sent out from the MTI.

PE2 encapsulates the Join message with GRE, reverts it to a
common multicast data packet (11.11.2.1, 239.1.1.1) on the public
network with the address of the IBGP interface on PE2 being the source
address and the share-group address being the group address. PE2 forwards
the multicast data packet to the public network instance on PE2 for
forwarding.

The multicast data packet (11.11.2.1, 239.1.1.1) is forwarded
to the public network instance on each PE along the Share-MDT. PEs
decapsulate the packet and revert it to the Join message sent to the
VPN RP. The PEs then check the Join message. If the VPN RP (CE1) is
their directly connected sites, the PEs send the message to the VPN
instances on them for further processing. Otherwise, the Join message
is discarded.

After receiving the Join message, the VPN instance on PE1 considers
that the message is received from an MTI. The instance creates the
(*, 225.1.1.1) entry, and specifies an MTI as the downstream interface
and the interface towards CE1 as the upstream interface. Then, the
instance sends the Join message to the VPN RP.

After receiving the Join message from the instance on PE1,
CE1 updates or creates the (*, 225.1.1.1) entry. The multicast shared
tree across VPNs is thus set up.

Process of Transmitting
Multicast Data Packets

When a VPN runs PIM-DM, VPN multicast
data is transmitted across the public network along the VPN SPT.

When a VPN runs PIM-SM,

If receivers and the VPN RP belong to different sites, the
VPN multicast data is transmitted across the public network along
the VPN RPT.

If the multicast source and the receiver belong to different
sites, the VPN multicast data is transmitted across the public network
along the source tree.

In the following example, the public network and VPNs run
PIM-DM. VPN multicast data is transmitted across the public network
along the SPT. An example is given to show the process of transmitting
multicast data packets along the Share-MDT.

As shown in Figure 7-11, the multicast source
in VPN A sends multicast data to the group G (225.1.1.1). The receiver
belongs to Site2 and is connected to CE2.

Figure 7-11 Process of transmitting multicast data packets

The process of transmitting VPN multicast data across the
public network is as follows:

The source sends VPN multicast data (192.1.1.1, 225.1.1.1)
to CE1.

CE1 forwards the VPN multicast data to PE1 along the SPT. The
VPN instance on PE1 searches for the forwarding entry. If the outgoing
interface of the forwarding entry contains an MTI, the instance forwards
the VPN multicast data to the related P for further processing. The
VPN instance on PE1 then considers that the Join message is sent out
from the MTI.

PE1 encapsulates the VPN multicast data with GRE and reverts
it to a public network multicast data packet (11.1.1.1, 239.1.1.1)
with the address of the IBGP interface on PE1 being the source address
and the share-group address being the group address. PE1 then forwards
the multicast data packet to the public network instance for forwarding.

The multicast data packet (11.1.1.1, 239.1.1.1)
is sent to the public network instance on each PE along Share-MDT.
Each PE decapsulates it, reverts it to VPN multicast data, and forwards
it to the related VPN instance for further processing. If there is
an SPT downstream interface on the PE, the data is forwarded along
SPT. Otherwise, the data is discarded.

The VPN instance on PE2 searches for the forwarding entry and
then sends the VPN multicast data to the receiver. So far, the process
of transmitting VPN multicast data across the public network is complete.